I tried using a mouse in my left hand in left handed mode back in the day. Eventually, I got tired of changing the settings on shared computers at home/school, and I realized it wasn't any more or less hard to use the mouse with my right hand. Today, I use a right handed vertical mouse. Interestingly, I play guitar right handed, despite being inspired to play by Kurt Cobain. My girlfriend got me left handed scissors as a joke, but man, they actually feel better.
My sister used to use a neutral mouse, with her left hand, on the left side of the keyboard but in "normal" or "right handed mode". Now she has a laptop with a big touch pad in the middle.
I am lefty and occasionally use the mouse left-sided when I have right arm injuries. What I don't do is swap the buttons. That way, nothing has to change, I can switch to the preferred side at any time, and coworkers aren't stymied when they need to drive the computer.
I do what your sister used to do. (Left hand mouse use, buttons left at default).
Even game that way. Started using it that way as a kid, by the time I learned it was possible to switch the buttons that seemed less natural than leaving it as default.
Also made it much easier to use shared computers in school labs and the like.